If you are going to end up with a RUM, then start with one....you just may pick up a "shooter"!!!
Also, and I could be wrong, but I remember seeing somewhere that there are some feeding issues, magazene work, or something....that needs to be done to switch from a regular Mag to a RUM. Some of you gun cranks can answer that question.
If its true, avoid the headache and do it right the first time.
Just my opinion.